The compatibility layer is overhead, but the key difference for many games is that DXVK swaps directX for Vulkan, and Vulkan often gets better performance.

The performance gains of using steamOS are twofold, there's less OS load (this is particularly noticeable in low performance games, windows will consume much more battery on a game like Dead Cells than SteamOS will), and there's also a vulkan performance increase for some games. My understanding is if you see a big performance increase in a demanding game, that's usually thanks to vulkan.

There is overhead but Vulkan allows you to batch draw calls in a far more efficient manner. It can also generally use multi threading to feed a GPU even if the game isn't coded with that in mind. Basically Vulkan offers so many improvements to efficiency and parallelization that the overhead is a drop in the bucket compared to the overall speedup in draw call optimization alone.

entirely

It's not just overhead.

In the same way that talking to a presidential translator is faster for a diplomat than talking to Trump. The translation layer can communicate more concisely and effectively.